Stable zero electronic circuitry



Oct. 29, 1963 H. D. MORRIS ET A1.

STABLE ZERO ELECTRONIC CIRCUITRY Filed April 1960 4 Sheets-Sheet 1 I-H IIMIHWH Oct. 29, 1963 H. D. MORRIS ET A1. 3,109,145

STABLE ZERO ELECTRONIC CIRCUITEY Filed April 27, 1960 4 Sheets-Sheet 2 COMMON MODE C UEEEA/T 20 Z4 PADDLE sPAc/NG /N M/LL/-1Nc/-res OUTPUT l/OLTS W/TH LOAD EE5/5TE2= UTPUT VOLTE WITH /OK LOAD J:- I lEl 2n IN VEN TORS ATIDPA/EYS Oct. 29, 1963 H. D. MORRIS ET AL 3,109,145

STABLE ZERO ELECTRONIC CIRCUITRY Filed April 27, 1960 4 Sheets-Sheer No 0.40 ourPur vous MON MODE CUEEEJVT' P40p/ 6 sPAc/,va w M/u-f/vc/fs I IED 4 JOSEPH Z'ack/A/GHAM JNVENToR.

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Oct. 29, 1963 H. D. MORRIS ET AL STABLE ZERO ELECTRONIC CIRCUITRY 4 Sheets-Sheet 4 Filed April 27, 1960 lmll IIHIH-HIIMIH Hamm 0. Moe/ws Josep# ZBac/f//vG//AM IN V EN TORS United States Patent O Sl E ELECTRNH@ QlRClUlTlFV l'rlarold Pleasant Hills, and Joseph l. Buckingham, Concord, Calii., assignors, by mesne assignments,

This invention relates to stable-zero electronic circuitry and more particularly to such stable-Zero electronic circuitry for use in accelerometers.

Difficulty has been experienced wit-h circuitry heretofore provir ed for accelerometers such as the circuitry disclosed in copending application Serial No. 794,487, iiled February 1959, because of the change in the Zero spacing of the position of the pickol' as a function of temperature. lt has been found that in such circuitry the Zero shifted because the oscillator in the circuitry osciilates at a point which is determined by the gain of the amplifier in the circuitry and the amount of feedback to the oscillator as well as other reasons. For example, there is a substantial change in the Zero position of the paddle `because of `temperature changes `which primarily affect tue vacuum tubes and transistors utilized in the circuitry. As is well known to those skilled `in `the art, the beta gain of transistors is particularly inilucnced by temperature changes. Thus, if for any reason the output of the oscillator is less, it is necessary for the paddle to move in order Ato obtain the same output from the oscilla` tor. This, therefore, represents a change inthe steady state position of the paddle which reilects itself in an accelerometer as a change in the zero output or the sensing axis. There is, therefore, a great need for stablezero electronic circuitry.

ln general, it is an object of the present invention to yprovide electronic circuitry which has a stable electrical zero throughout a wide temperature range.

Another object of the invention is to provide electronic circuitry of the above character' which when used with an accelcrometer will provide ya stable axis of sensitivity for the accelerometer throughout wide temperature ranges.

Another object of the invention is to provide electronic circuitry of the above character in which a single oscillator is used.

YAnother object of the invention is to provide electronic circuitry of the above character in which means is provided for regulating the level at which the oscillator oscillates.

Another object of the invention is to provide electronic circuitry of the above character in which the output of the circuitry is essentially zero when the paddle is in a mid-point position between two picltoiis, -for wide temperature ranges.

Another object of the invention is to provide electronic circuitry of the above character in which symmetrical picltoils and symmetrical connecting `circuitry are utilized.

Another object of the invention is to provide electronic circuitry of the above character in which the differences between the outputs of the two symmetrical sides of the circuitry are utilized as the useful `output from the electronic circuitry.

Additional objects and features of the invention will appear from the following description in which the preerred embodiment has been set forth in detail in con* junction with the accompanying drawings.

Referring to the drawings:

FEGURB l is a block diagram of the electronic circuitry incorporating our present invention.

FIGURE 2 is a detailed circuit diagram of the electronic circuitry incorporating our invention.

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FIGURES 3, 4 and 5 are graphs showing output characteristics of the circuitry shown in FIGURE 2 at 70 C., 25 C., and -20 C., respectively.

`ln general, our electronic circuitry consists of an oscillator and a pair of spaced pickofts which are fed by the oscillator. A moving element is disposed between the picliots and is movable between the pickoils to modify the iields between the picltoils. Movement of the paddle serves to vary the outputs from the pickolls. Detecting means detects the output of each ci the pickolis. Summing means is connected to each of the pickolis for algebraic summing of outputs from the detecting means. Negative feedback means is connected to the summing means and to the oscillator to regulate the level of oscillation of the oscillator to thereby maintain the output of the summing means at a constant level when the paddle is in a predetermined position with respect to said pair of pickolis. Our electronic circuitry, yas `shown in block diagram form in FlGURE 1, consists of an oscillator l1 which drives two separate channels, channel A and chan nel B. Each of the channels consists of a pickoli l2, the output of which is supplied to a detector 1.3 and amplied by an amplier ld. The outputs of the two channels are connected into a summing network le which is connected to an output terminal t7. A negative feedback circuit lil is connected between the summing network lt and the oscillator' lll.

From the block diagram, `it can be seen that the oscillater drives the pickolts l2 in `both channels. rllhe pickoils need not be, but preferably are of the resonant type described in our copending application Serial No. 794,487, filed February 4, 1959. The two ampliliers lll are of a complementary nature so that their drift characteristics are subtractive. T his is advantageous because this causes cancellation in the summing network llo of :any change in the ampliiiers which is of a type likely to atleet both amplifiers in the same fashion. Essentially, the two pickoils are driving two channels that are subtracted in the output.

A moving element in the form of a paddle it@ is mounted between the two picltofs l2 and is movable between the two pick-oils to modify the field between the pickoils. rPhe output is normally adjusted `so that it is zero when the 'paddle il@ is in a mid-point position `between the two pickoils. lf the paddle l@ is moved so that it is closer to the piel/toil in channel A, it will reduce the output from that picltolt and increase the output of the picltotl in channel B to provide a dierential output at 1.7 which will be either negative or positive depending on which way the paddle is moved.

The feedback circuit lil provides a closed loop which causes the oscillator lil to operate at a level which is necessary in order to maintain the desired output from the circuitry. This means that the same output or effort will be available from the circuitry as the paddle 1l9 moves lback and forth between the two pickotls regardless of the temperature at which the circuitry is operating. For example, when the ampliers le are operating at high gain, the output of the oscillator will be reduced until the output at i7 is of the proper amplitude. ln the same way as the gains of the amplifiers ld drop for any reason, as for example, because of low temperatures, the amplitude of the oscillator will rise until there is suihcient output from the ampliliers to provide the desired output.

A detailed circuit diagram incorporating our invention is shown in FIGURE 2 in which the oscillator is of the transistor type and includes a transistor Til which has a tank circuit and a feedback circuit. A transformer 2l is also provided and has four windings 22, Z3, 24 and 25. The windings, as shown, are wound on a toroidal core 26.

The winding 22 of the transformer 21 and the capacitor C1 form the tank circuit for the oscillator and are connected to the collector of the transistor T1. The B- and B+ power supplies are connected to a pair of Zener diodes D1 and D2 and a resistor R1, and serve to provide a reference voltage supply for the base of the transistor T1. The capacitor C2 connected across the diodes D1 and D2 serves as a filter.

The frequency of the oscillator is primarily determined by the coil 22 and the capacitor C1, and their values are chosen so that the oscillator operates at a desired frequency. The exact frequency of oscillation of the oscillator is not important. However, it is preferable that the oscillator 'operate at a frequency of approximately 2 megacycles per second.

The feedback for the oscillator is provided by the winding 23 which is connected between the emitter of the transistor T1 and the base of the transistor through a coupling capacitor C3. A resistor R2 connects the emitter to the B power supply and serves to provide the collector current for the oscillator. This resistance is chosen so tha-t the transistor has enough gain to oscillate due to the feedback from the winding 22 provided by the winding 23.

Each of the pickoffs 12 is symmetrical and consists of two windings 31 and 32 which preferably are woundso that the paddle or vane 19 has the most effect upon the coils. For that reason, it is generally preferable to wind the pickoffs so that the coils 31 and 32 are coaxial and lie in generally the same plane as, for example, in the form of pancake-type coils. As shown, the two windings 31 of the pickoffs are serially connected and are fed by the winding 23 of the transformer 21. Although the serial connection is not absolutely necessary, it helps to give a slight increase in gain in the circuitry because when one pickoff is loaded due to paddle movement, the Signal in the other pickoff is increased. This gives a differential effect and is additive when you move the paddle 19 toward one of the pickofis and away from the other.

A capacitor C4 is connected across each of the Windings 32 and serves to provide a tuned circuit for a purpose hereinafter described. It should be pointed out that both of the pickoffs 12 are identical. One side of each of the windings 32 is connected to ground as shown. A portion of the output of each of the pickoffs is fed through a coupling capacitor C to the detectors 13. Each detector 13 consists of diodes D3 and D4. A capacitor C6 is provided across each pair of diodes and serves as an RF filter. The outputs of the diodes are supplied to the amplifiers 14 which are similar.

The amplifier 14 in the upper channel, or A channel, can be called a pull-up amplifier. It is formed of transistors T2 and T3 of the P-N-P type. When current is fed to the detector in channel A, current flows through the load to the B+ supply. Two such transistors are provided so that there is adequate gain. The amplifier 14 in the lower channel, or B channel, can be called a pull-down amplifier. It is formed of transistors T4 and T5 of the N-P-N type. When current is fed to the detector in channel B, current flows through the load to the B- supply.

In constructing the amplifiers, each pair of transistors is chosen so that they are identical and then each pair of transistors is matched to the other pair so that they have the same beta characteristics with respect to temperature.

The output current from the amplifiers 14 flows into the summing network 16 which consists of serially connected resistors R3 and R4 with a capacitor C7 connected across them. The total current in each of the resistors R3 and R4 provides a total voltage which is fed back to the serially connected windings 24 and 25 on the toroidal core 26 through the Zener diodes D5 and D6, and is utilized for regulating the output of the oscillator as hereinafter described. The differential output from the summing 4 network is supplied from a junction between the resistors R3 and R4 to the output terminal 17.

Operation of the electronic circuitry shown in FIG- URE 2 may be briefly described as follows. The oscillator 11 operates in a more or less conventional manner in which the frequency of the oscillator as `hereinbef'ore described is determined by the value of the capacitor C1 and the winding 22. Feedback for continuing oscillation is provided by the winding 23. The pickoffs 12 are of the resonant type described in our copending application Serial No. 794,487, led February 4, 1959, in which movement of the paddle 19 towards one of the pickofs modifies or destroys the coupling between the coils or windings 31 and 32 and loads the coil 32. The primary effect, however, as described in our copending application, is the detuning of the tuned circuit consisting of the winding 32 and the capacitor C4.

In utilizing the resonant pickofr` with our stable-zero electronics, it was found that the best results were 0btained by tuning the pickoif so that the output from the pickoff was at a maximum when the paddle or vane was adjacent the other pickoff. This, in effect, is tuning the resonant pickoff for the maximum possible spacing of the paddle 1@ from the pickolf and then normally operating the pickoff at a point which is equidistant between the two pickotlis.

When the paddle 19 is in the mid-point position, the output from each of the pickoifs to the detectors should be equal and this equal output is supplied to the arriJ plifiers 14, the output of which is applied to the surnming network 16. The summing network gives us the difference between the currents flowing in the transistors in the upper and lower amplifiers and which if equal, gives an output which is zero. When the paddle 19 is moved towards the upper pickoif 12, the current flowing in the detector and the amplifier in channel A is reduced which decreases the positive current iiow to give a negative output at the output terminal 17. Conversely, when he paddle 19 is moved towards the lower pickoff 12, a positive output is provided at the terminal 17.

When something occurs to change the output of the amplifiers as, for example, if the temperature is increased and the beta of the amplier transistors rises, additional output current will flow through the resistors R3 and R4 which will be applied through the feedback circuit 18 to the windings 24 and 25 which, in turn, will cause a decrease in the feedback from the collector circuit of the oscillator to the emitter to cause the oscillator to operate at a lower level. The overall effect is, therefore, the same as a negative feedback circuit which tends to maintain the same total current flow in the output.

The resonant pickolts have made it possible to obtain sufficient signal strength even though relatively wide padidle spacings are utilized and also to permit reduction in the signal to noise ratio in the output of the detector.-

If, for some reason, the oscillator should suddenly operate at a level higher than it normally has been operating, then the current in both of the amplifiers would also increase in a proportionate amount. The voltage developed across the resistors R3 and R4 would accordtingly be increased which would be applied across the windings 24 and 25 to cut down on the amplitude of oscillation of the oscillator by reducing the feedback to the oscillator.

The feedback circuit 11S, however, is not effective for very small voltages because the diodes D5 and D6 do not begin conducting until a predetermined voltage is placed across the same. Thus, until this predetermined voltage is created by the resistors R3 and R4, nothing will happen to prevent the oscillator from increasing its amplitude until this predetermined voltage has been developed. However, as soon as this predetermined volt-l age has been reached, one of the diodes begins conduct-v ing, causing current to flow in the windings 24 and 25 to creat-e losses which become greater and greater as the amplitude of the oscillator rises until the oscillator can no longer increase its amplitude. Thus, the feedback circuit acts as a shunt regulator type of ycontrol in which the oscillator is almost unloaded until the control point is reached, after which the control or regulator windings and 25 throw in a heavier and heavier load on the oscillator until the oscillator stops rising in output voltage.

In the same manner, if the characteristics of the amplifiers le `change so that they are putting out substantially less than they were previously putting out for the same amount of drive, the voltage across the resistors and Rd would again drop and one of the diodes D5 and De would no longer conduct and the load would drop oil of the transistor oscillator and the amplitude of the oscillator would begin rising until there was suf- {icient current flowing in the two ampliers to `drive the resistors and Rd up to the `same total voltage again. Thus, we have in essence a closed loop which causes the oscillator to continuously run at a level which is necessary to give the proper output current in the output stage. This means that there will always be the same efrort available as the conducting plane l@ moves back and forth in order to differentially aiiect the amplifiers. There is also the same eiiort available in the output circuit no matter what the temperature at which the circuitry is operating because the oscillator, when the ampliiiers are operating at high gain, will reduce the arnplitude until the current is proper. When the gain of the amplifiers decreases, the oscillator amplitude will rise until there is sufficient output current.

By way of example, one electronic .circuit `constructed in accordance with the following description had the iollowing components and values:

D5 and Do-Type 1N754 This electronic circuitry was `found to operate very satisfactorily and output characteristics thereof are shown in FIGURES 3, 4 and 5. ln these figures, the common-inode current and the output voltage are plotted as a function of paddle spacing, that is, the paddle distance from picltoff A. The :common-mode current was measured at the junction of the resistors R3 and R4 at no load. The picltolis A and B had a spacing of 50 Inilli-inches so that when the paddle was adjacent the picltoil A, there was a spacing of 50` mild-inches between the paddle and the piekoli B. Thus, at the zero position, the paddle was adjacent the pickoii A. With the paddle in this position, the output voltage at no load is shown by the curve 52 and with a 10K load is shown by the curve 531. At the zero paddle position, the output at no load was slightly over 13 volts, and with a 10K load was approximately 12 volts. `lt will be noted in FIGURE 4 that the voltage follows approximately a .straight line until the paddle has been moved about 9 milli-inches from pickoif A, at which time there is sufficient current ilow through the piclcoii A that a common-mode cunrent shown by the curve 1t begins to appear'. At this point, it will be noted that the output voltage begins to drop rather rapidly and that the common-snode current increases rather rapidly, up to approximately a paddle spacing of 111/1 to 111/2 milliinches. After that, the output voltage is substantially ilat and the common-mode current only increases gradually up to approximately 211/2 milli-inch spacing. This transistion at the 11% to the 111/2 nulli-inches indicates that at this point the voltage generated across the resistors R3 and Rd was suiiicient to overcome the voltage drop across the Zener diode to permit the regulator circuit to take over to control the output of the oscillator.

The common-mode current thereafter does not continue to increase at such a rapid rate because of the inliuence of this feedback circuit. The common-mode current continues to increase gradually because the regulator circuit has a -iinite gain. At a spacing of approximately 20 nulli-inches, a point is reached where the currents in both channels A and i3 are substantially equal. At this point we are reaching the normal operating range of the electronic circuit, that is, with the paddle in a mid-point position between the two pick.- otfs. in this region, it will be noted that the output voltage in either a loaded or unloaded condition drops from approximately -l-lO volts to a volts. To obtain this great range in output voltage, it is only necessary for the paddle to move approximately one milliinch. During this time, it will be noted that the commonrnode current is substantially flat and is maintained so by the regulating feedback circuit. As the paddle is moved towards the pickoti E, output .curves and coinmon-mode current curves very similar to those produced when moving from the piokoft4 A to the mid-point position are produced. Although the common-mode current produces a bell-shaped curve, it is only the very central portieri which is iat and of particular interest because it is the normal operating range for the circuitry.

The curves shown in FIGURE 4 were obtained when the electronic circuitry was at 25 C. or room temperature. rl`ests were also conducted at other temperatures. In FGURE 3 are shown 4the curves for a test conducted at 701o C., and in FIGURE 5, the curves for a test conducted at `--20 C. in each of these iigures it will be noted that the paddle was at approximately the same position-2l5 milli-inches from picltoti A for zero signal. This clearly indicates that our electronic circuitry provides a stable zero in that the zero position for the paddle is at the same position between the piclcolis regardless of the temperature at which the circuitry is operating. The figures show that the paddle spacing for zero output at the various temperatures varied at most only a fraction of a milli-inch.

From the curves, it will be noted that the commonniode current changed more significantly at the `different temperatures. This was due primarily to the beta change in the transistors caused by the temperature changes. At a very cold temperature, such as at 20 C., the beta of the transistors dropped to a low value so that regulation was more diiiicult. Substantial regulation, however, was still taking place and the full output range was being provided.

It is apparent from the foregoing that we have provided a new and improved electronic circuitry which makes it possible to lobtain `a stable zero through wide ranges of temperature. As pointed out previously, this is particularly important in certain applications as, for

example, for use with accelerometers where it is desirable to m-aintain a stable aXis of sensitivity and a stable zero output for the accelerometers.

We claim:

1. .In electronic circuitry of the character described, two channels each consisting of a pickoff and a detecter, common oscillator means feeding the pickoff of each of the channels, means for summing the output of the channels, and feedback means connecting the summing means to the oscillator means to regulate the output from the electronic circuitry.

2. Circuitry as in claim 1 wherein s-aid pickoffs are spaced apart a predetermined distance and wherein a moving element is disposed between the pickoffs and is movable from one pickoff to the other to modify the lield between the pickoifs.

'3. -In electronic circuitry 0f the character described, a pair of channels, each channel consisting of a pickotf, a detector connected to said pickotf and a D.C. amplier connected to said detector, said pickolfs being spaced apart a predetermined distance, a moving element disposed between the pair of pickolfs and movable between the same to modify the field between the pickoffs, a cornmon oscillator vfor driving said pickoifs, said moving element serving to vary the output from the pickoffs as the moving element is moved between the pickoifs, means lfor summing the outputs of the channels, and feedback means measuring the summed output of the channels and supplying a portion of the summed output to said oscillator to maintain constant output capability of the circuitry.

4. Electronic circuitry as in claim 3 ywherein said pickoff is of the resonant type.

5. In electronic circuitry of -the character described, a pair of spaced pickots, ia moving element disposed between the pickoffs and movable between the same, a single common oscillator ydriving the pair of pickofs, detector means connected to each of the pickoffs, D.C. amplifier means connected to each ofthe detectors, summing means for summing the outputs of the amplifiers, feedback means for measuring the summed output from both amplifiers and connected to the oscillator to maintain constant output capability of the circuitry.

-6. In electronic circuitry of the character described, a

gpair of spaced pickots, each of the pickoffs having at least ltwo windings, common oscillator means for driving one of the windings of each of the pickoffs, a moving element disposed between the pickoifs and movable between the same, detector means connected `to the other winding of each of the pickoifs, D.C. amplifier means connected to each of the detector means, means for summing the Output of the D.C. amplifier means, Aand means forV measuring the summed output of the amplifier means and feeding the same back to the oscillator to regulate the output capability of the circuitry.

7. Electronic circuitry as in claim 6 wherein a capacitor is connected across the said other winding of each of the pickoifs to provide a tuned circuit.

8. In electronic circuitry of the character described, a transformer having first, second, third and fourth windings, an oscillator including a tank circuit and a feedback circuit, the first winding of the transformer being connected into the tank circuit of the oscillator, the second winding of the transformer being connected into the feedback circuit of the oscillator, a pair of spaced pickoifs, each of the pickoffs having at least a driving winding and an output winding, means connecting the third winding of the transformer to the `driving winding of each of the pickoifs, a moving element disposed between the pickoifs and movable between the same, detector means connected -to each of the output windings of the pickoffs, amplifier means connected to each of the detector means, means for summing the output of the amplifiers, and feedback means connected between the summing means and the third and fourth windings of the transformer, said feedback means serving to maintain the total current ow in the amplifiers so that the output from the summing means tends to be zero when the moving element is at a midpoint position between the pickoifs.

9. Electronic circuitry as in claim 8 together with a capacitor connected across at least one of said windings of each of said pickoifs to provide tuned circuits.

References Cited in the le of this patent UNITED STATES PATENTS 2,153,986 MfacLaren Apr. 1l, 1939 2,408,524 Mestag Oct. 1, 1946 2.614,163 Roper Oct. 14, 1952 

1. IN ELECTRONIC CIRCUITRY OF THE CHARACTER DESCRIBED, TWO CHANNELS EACH CONSISTING OF A PICKOFF AND A DETECTOR, COMMON OSCILLATOR MEANS FEEDING THE PICKOFF OF EACH OF THE CHANNELS, MEANS FOR SUMMING THE OUTPUT OF THE CHANNELS, THE FEEDBACK MEANS CONNECTING THE SUMMING MEANS TO THE OSCILLATOR MEANS TO REGULATE THE OUTPUT FROM THE ELECTRONIC CIRCUITRY. 